1. Field of the Invention
The present invention relates to a portable precision caliper, or slide gauge, used for measuring linear dimensions. More specifically, the present invention relates to a portable precision caliper of an electronic type.
2. Related Art
Traditional vernier calipers tend to be gradually supplanted by electronic calipers which afford better reading at a competitive price. Most of the electronic calipers commonly used nowadays incorporate capacitive sensors. The variations in capacitance caused between an array of electrodes on the caliper's shaft and an array of electrodes disposed opposite on the caliper's slide are measured so as to supply information depending on the position of the slide along the shaft. This information is displayed on a screen, usually integral with the slide. Circuits of this type are described, for example, in patent application EP96810686 in the name of Brown & Sharpe TESA SA.
The principle of capacitive measuring has established itself on the one hand because it provides excellent resolution and precision, and on the other hand because the electricity consumption of these devices is very low, so that they can be powered by a battery for example. Such sensor calipers of a capacitive type must however be kept clean for them to function properly. They are thus badly suited to functioning in a humid environment or an environment subjected to splashes of lubricant or to dust for example. In such difficult conditions, hand-operated vernier calipers or watch-type calipers are therefore preferred to electronic calipers.
Document EP286820 describes a caliper comprising a scale fitted with magnetized sections. A magnetic reading head is fitted on the slide and records the variations in magnetic field caused by the displacement of the slide. Such a magnetic head is however relatively costly and voluminous, so that it is difficult to integrate it within the reduced volume of a caliper. The accuracy of measurement depends directly on the precision and noise in the magnetic head as well as on the correct positioning of the head in relation to the scale. Furthermore, the resolution afforded by this device is limited.
One object of the present invention is therefore to propose a portable electronic caliper which is improved with respect to the prior art devices. In particular, an object of the present invention is to propose a caliper comprising a sensor with a low sensitivity to dust and lubricants, providing a resolution on the order of a hundredth of millimeter and an accuracy on the order of a few hundredths of millimeter, that is, a performance which is comparable to that afforded by capacitive calipers at similar cost, and with an electricity consumption sufficiently reduced so as to be able to be powered by a simple battery, for example by a lithium battery.